The present invention relates to detecting and reducing adjacent channel interference in a radio receiver, and more specifically, to determining the presence of an interfering upper adjacent channel or lower adjacent channel and shifting the frequency of a mixing signal to reduce adjacent channel interference.
Commercial AM (amplitude modulation) and FM (frequency modulation) broadcast bands include a plurality of evenly spaced channels. Broadcast stations are allocated a channel for broadcasts within an assigned frequency range. The power spectrum of a transmission depends on the energy content of a radiated signal at each frequency. While most energy in a transmission can be limited to an assigned channel, some radiated energy will be at frequencies outside the assigned channel. This radiated energy can manifest itself as noise in an adjacent channel. The noise can include ultrasonic noise (USN) and wide band amplitude modulation (WBAM).
Assignment of broadcast channels to transmitters has been determined according to geographic location and other factors to minimize interference (noise) between transmission of adjacent channels. However, in highly populated areas there is frequently a limited number of available channels. In this situation, radio receivers must often cope with strong signals on adjacent channels which create signal components in the desired channel. These signal components interfere with the reception of the desired signal. Interference has traditionally been considered objectionable when the total power in an adjacent channel signal is about 30 dB greater than the total power in the desired channel signal.
Prior art receivers have detected the presence of objectionable adjacent channel signals by various methods. These methods have included separately tuning each channel and measuring its signal strength, detecting beat components caused by adjacent channels and detecting the difference in signal levels of a narrow band portion of the desired channel and the full band of the desired channel. In these receivers, when adjacent channels were not objectionable, a wide band intermediate frequency (IF) filter was used to maximize desired signal quality. When an adjacent channel was objectionable, a narrow band IF filter was switched into the signal path to eliminate adjacent channel interference. However, introduction of the narrow band IF filter into the signal path introduced distortion into the desired signal and affected its quality.
Another prior art approach used an adjacent channel detector with a tri-band filter that filtered the IF signal to derive a lower adjacent channel signal, a desired channel signal and an upper adjacent channel signal. The signal levels of the three channels were then compared. If only one of the adjacent channels had a signal level greater than that of the desired channel, then adjacent channel interference reduction was initiated. This was accomplished by changing the frequency of a mixing signal coupled to an IF mixer so as to move away from the interfering adjacent channel signal. This effectively moved the interfering adjacent channel signal out of the IF pass band. The addition of variable bandwidth active IF filters, switchable IF filters or tri-band filters (to adjust the bandwidth of the desired channel based on the level of interference of adjacent channels) added additional cost to the receiver. Detection circuitry associated with the tri-band filter approach further increased part count and cost of the receiver.
The present invention provides a technique for reducing adjacent channel interference in a processor controlled FM receiver. Initially, the technique determines whether the desired channel has interference. If so, signal information on adjacent channels is collected. The desired channel is then shifted away from one of the adjacent channels when the collected signal information indicates that only one of the adjacent channels is appreciably interfering with the desired channel. An advantage of the present invention is that it allows for a reduction in adjacent channel interference, in receivers that include alternate frequency (AF) switching capability, without requiring additional components.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.